What Is The Best Way to Turn Plants into Energy?

A new study compares biofuels with bioelectricity
David Biello Scientific American 7 May 09;

The environmental case for ethanol from corn continues to weaken. Turning the food crop into ethanol would not be the best use of the energy embedded in the kernels' carbohydrates, according to a new study in Science. That's because fermenting corn into ethanol delivers less liquid fuel energy for internal combustion engines than does burning the kernels to generate power for electric motors.

"We had been studying the area of land that would be available to grow crops for energy and we were curious to discover the most efficient use of these crops," explains environmental engineer Elliott Campbell of the University of California, Merced, who led the study. "We found that with a given amount of biomass you could produce more transportation and greenhouse gas offsets with electricity than with ethanol."

The new study shows that burning biomass to produce electricity rather than converting it to ethanol (made from corn kernels or the other parts of the plant, so-called cellulosic ethanol) delivers 81 percent more miles per acre of transportation in electric vehicles than ethanol burned in internal combustion, even taking into account the lifetime costs of the expensive batteries available today. "The input energy to produce an electric vehicle was 1.5 times the energy to produce an [internal combustion vehicle]," Campbell says. "The batteries currently require large energy inputs in the vehicle production component of our life cycle assessment."

On average, looking at a wide variety of source crops (corn kernels to switchgrass), ways to convert plants to energy, and vehicle sizes (ranging from compact cars to SUVs), bioelectricity delivered 56 percent more energy for transportation per acre, even including the fact that making ethanol produces other useful products, such as cattle feed. To take just one example: a small truck powered by bioelectricity could travel almost 15,000 city and highway miles (24,000 kilometers) compared with just 8,000 comparable miles (13,000 kilometers) for an internal combustion equivalent.

From the atmosphere's point of view, growing biomass to burn in a power plant and using the electricity to move a car avoids 10 tons of carbon dioxide emissions per acre, or 108 percent more emission offsets than ethanol. "One other aspect of the electricity pathway is that most emissions are concentrated in one location, which provides perhaps an opportunity for more control of the emissions," Campbell notes. "It also perhaps locates [other air pollution] emissions in a place where impacts might not be as harmful as where cars are driven today."

Of course, such a bioelectricity future for transportation would also rely on widespread availability of cars and trucks with batteries and electric motors. "A great deal of innovation must happen in vehicle and power transmission technologies to make that a reality," argues Renewable Fuels Association spokesman Matt Hartwig, an ethanol trade association that owns an ethanol-electric hybrid car. "In the meantime, Americans still need liquid transportation fuels. If the goal is to have more of those gallons come from renewable sources rather than imported oil, fuels like ethanol are the only technologies that are having an impact today."

He adds: "In theory, you could have a plug-in hybrid with a renewable fuel powered [internal combustion engine] and eliminate the need for petroleum all together."

The Obama administration seems to agree, granting $786 million in 2009 for biofuels research and setting up the Biofuels Interagency Working Group to study how best to meet the renewable fuel standard mandated by Congress that will require increasing the amount of renewable fuels, such as ethanol, to 36 billion gallons by 2022.

But the U.S. Environmental Protection Agency (and the California Air Resources Board) have noted that turning corn into ethanol can actually be a significant source of greenhouse gas emissions and other unintended environmental effects, largely by driving the expansion of agriculture and its attendant pollution—as evidenced by previous studies published in Science.

All use of biomass—whether for ethanol or electricity—runs the risk of displacing food crops, however, as well as the need for large amounts of water. "Both pathways could be totally disastrous if these types of impacts can't be avoided," Campbell admits. "This is going to be a constrained area of land and amount of biomass, so how much transportation and greenhouse gas offsets can we milk out of this constrained land? It looks like the electricity pathway might get us more bang for the buck."

And burning biomass for electricity while capturing the CO2 emissions from such a power plant can actually result in carbon-negative power generation—taking CO2 out of the atmosphere. "By sequestering the flue gas CO2 at the power plant, the bioelectricity pathway could result in a net removal of CO2 from the air," the researchers wrote, and that could help with the problem of ever-rising levels of the greenhouse gases causing climate change.